The energy selection of charged particles, e.g. electrons, is effected preferably by electrostatic deflection systems. Their effects depend upon the different degrees of deflection of particles with different energies which enables the discrimination against particles of undesired energies. Advantageous electrostatic energy filters which have been provided heretofore are predominantly the cylindrical mirror, the spherical deflector and the cylindrical deflector which have found widespread use in practice, although basically planar deflecting plates can be used as well. Theoretically the toroidal deflector has also been investigated (Hermann Wollnik, Optics of Charge Particles, p. 119, Academic Press, Orlando, 1987).
All of these mentioned energy filters are characterized by appropriate dimensioning to provide at least first order angular focussing in the energy dispersion plane. Depending upon the geometry of the filter chosen, these dispersion planes form a family of planes which are parallel to one another as in the cylindrical deflector, or are inclined to one another, as in the toroidal and spherical deflectors or in the cylindrical mirror. The spherical deflector and the cylindrical mirror also have the especially advantageous stigmatic focussing.
The angular focussing enables a focussed transport of charged particles with a solid angle different from zero through the energy filter. The magnitude of the admissible solid angle is, however, limited by image aberrations, especially angular aberrations. As a result, the energy filtering is poorer for particles arriving out of a larger solid angle. The admissible solid angle for the arriving particles must therefore be restricted by apertures. In an analogy to light optics, one has a limitation of the luminosity due to the image aberrations. For cylindrical, toroidal and spherical deflectors, the smallest nonvanishing angular aberration in the dispersion plane is of the second order in the angle, whereas for the cylindrical mirror of suitable construction, the first nonvanishing angular aberration is of third order.
Thus, with reference to the angular aberrations, the cylindrical mirror is more advantageous than the deflectors. On the other hand, deflectors enable the use of input and output slits with the energy filtering being, to a first approximation, independent of the slit height. With a cylindrical mirror, radially symmetrical hole apertures must be used as input and output apertures. Depending upon the application, therefore, either the cylindrical mirror or the deflectors can have advantages and will be preferred.
For the so-called electrostatic toroidal condenser of generally cylindrical basic configuration, a correction of the second order angular aberration in the dispersion plane has already been provided (DE-PS 26 20 877) by appropriate opposing curvatures of the deflection plates perpendicular to the dispersion plane. In this arrangement, an axial curvature of the potential characteristic in the region of the central beam is excluded (R.sub.e =.infin.). This means that the described deflector has no focussing effect perpendicular to the dispersion plane for a radiation bundle around the central beam. The drawback of this concept thus resides in a limitation of the usable solid angle; especially, this system has the disadvantage of nonstigmatic focussing, as in conventional toroidal deflectors or cylindrical deflectors.
Up to now, moreover, it has not been possible to determine the influence of the fringe field distortion from the input and output apertures upon the elimination of the second order angular aberration, since the calculations are of an analytical nature and are based upon an ideal toroidal field.
An effort to transfer the described possibility of eliminating the angular aberration to the spherical deflector leads to a loss of the stigmatic focussing of this system, since the stigmatic focussing of the spherical deflector results from the spherical symmetry thereof and the transfer of the described configuration to the spherical deflector results in a deviation from the spherical symmetry.